Racism is usually defined as views, practices and actions reflecting the belief that humanity is divided into distinct biological groups called races and that members of a certain race share certain attributes which make that group as a whole less desirable, more desirable, inferior or superior.

The exact definition of racism is controversial both because there is little scholarly agreement about the meaning of the concept "race", and because there is also little agreement about what does and doesn't constitute discrimination. Critics argue that the term is applied differentially, with a focus on such prejudices by whites, and defining mere observations of racial differences as racism. Some definitions would have it that any assumption that a person's behavior would be influenced by their racial categorization is racist, regardless of whether the action is intentionally harmful or pejorative. Other definitions only include consciously malignant forms of discrimination. Among the questions about how to define racism are the question of whether to include forms of discrimination that are unintentional, such as making assumptions about preferences or abilities of others based on racial stereotypes, whether to include symbolic or institutionalized forms of discrimination such as the circulation of ethnic stereotypes through the media, and whether to include the socio-political dynamics of social stratification that sometimes have a racial component. Some definitions of racism also include discriminatory behaviors and beliefs based on cultural, national, ethnic, caste, or religious stereotypes.

Racism and racial discrimination are often used to describe discrimination on an ethnic or cultural basis, independent of whether these differences are described as racial. According to the United Nations convention, there is no distinction between the terms racial discrimination and ethnic discrimination, and superiority based on racial differentiation is scientifically false, morally condemnable, socially unjust and dangerous, and that there is no justification for racial discrimination, in theory or in practice, anywhere.

In history, racism has been a major part of the political and ideological underpinning of genocides such as The Holocaust, but also in colonial contexts such as the rubber booms in South America and the Congo, and in the European conquest of the Americas and colonization of Africa, Asia and Australia. It was also a driving force behind the transatlantic slave trade, and behind states based on racial segregation such as the USA in the nineteenth and early twentieth centuries and South Africa under apartheid.
Practices and ideologies of racism are universally condemned by the United Nations in the Declaration of Human Rights.